Browse datastore should be enough for this monitor to work properly. We saw similar issues when system locale on the collector server or SCOM server is set to a non-English locale and specificly to a locale which uses comma(,) as a decimal point separator, could you check which locale is configured on your collector server and on SCOM server? Also could you check 'freePct' and 'freeGB' performance metrics for this datastore, let us know if you also see 0 there?
Most likely it's an issue with locale(with comma as a decimal point separator), please let us know if our assumption is correct. You can fix this issue by changing decimal point separator in regional settings (Control Panel > Region > Advanced Settings). Make sure you've changed this for account under which Veeam Collector service is running.
Let us know if this doesn't help or you have English locale on both servers.